Attachment studies using Chinese Hamster (CHO) cells suggest that the 69K OMP (pertactin) might play an important role in the adherence of bacteria to eukaryotic cells since B. pertussis mutants which are deficient in the production of pertactin adhere less well to CHO cells than wild-type bacteria. CHO cells were shown to attach to plastic wells coated with either purified pertactin or with the mammalian cell attachment protein fibronectin. Monoclonal antibodies reactive with pertactin inhibit CHO cell attachment to pertactin but not to fibronectin. Recent genetic analysis has shown that the sequence RGD, which is the cell binding site on fibronectin, occurs twice in the pertactin sequence. RGD-containing peptides inhibited CHO cell attachment to pertactin as well as to fibronectin, suggesting that CHO cell attachment to pertactin occurs through an RGD-containing site. Filamentous hemagglutinin (FHA) is another surface protein of B. pertussis which functions as an adhesin. Like pertactin it also contains an RGD sequence, however, and FHA-derived peptide containing the RGD sequence did not block attachment of CHO cells to FHA or other cell attachment proteins. Our studies suggest that FHA may contain a lectin site that interacts with carbohydrates present on the surface of mammalian cells. Recently, it has been shown that B. pertussis can invade and survive within mammalian cells. The RGD peptide from pertactin inhibits invasion but the RGD peptide from FHA does not. A monoclonal antibody (Mab) directed against the RGD region of pertactin also blocks invasion. These results suggest that pertactin may play a role in bacterial invasion.